Feedback on page SDL2_mixer/FrontPage: Links to SDL3

[nerd66]

Currently the links to playwave.c, playmus.c and LICENSE.txt link to the main branch, which uses SDL3. These should probably be updated to point to the SDL2 branch.
Made this an issue instead of editing myself because there are probably a lot more links to the main branch in other SDL2_Mixer wiki pages and I don't know which ones should be updated and which ones shouldn't.

[nerd66]

Just saw that the SDL2 branch doesn't have the two .c files, so just updating the links won't work.